A variety of spectroscopic, physical and kinetic methods are used to characterize the mechanism of electron transfer in important biological systems. Emphasis is placed on the enzyme complexes of the mitochondrial electron transfer chain and on the metalloflavoprotein, xanthine oxidase. Principal long-term objectives are to define the structural and dynamic attributes of the mitochondrial complexes in sufficient detail that hypotheses for the mechanisms of energy conservation can be rigorously evaluated. The principal techniques, optical, magnetic circular dichroism and electron paramagnetic resonance spectroscopy, will be employed in both equilibrium and kinetic experiments; nuclear magnetic resonance and Raman spectroscopy will also be utilized. Major issues to be addressed include: (a) The structural and kinetic properties of the components of cytochrome oxidase and the nature of the oxygen intermediates. (b) The mechanism of electron transfer in yeast Complex III and the validity of the Q-cycle. (c) Ligands of the heme centers in Complex III. (d) The nature of the iron-sulfur centers in Complexes II and III. (e) The mechanism of electron transfer in Complex II. (f) Analyses of the fundamentals of steady state kinetics in enzymes with non-interacting reaction sites for each substrate.